Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Photoluminescence (PL) imaging, performed for example using apparatus and methods disclosed in published PCT patent application No WO 2007/041758 A1 entitled ‘Method and System for Inspecting Indirect Bandgap Semiconductor Structure’ and incorporated herein by reference, has been shown to be of value for characterising silicon materials and devices, and silicon wafer-based photovoltaic (PV) cells in particular. As shown schematically in FIG. 1, photoluminescence 2 generated from a sample of a semiconductor material 4 with broad area photo-excitation from a source 6 of above-bandgap light 8 can be imaged with an image capture device 10 such as a camera or CCD array via collection optics 12, with the system preferably including homogenisation optics 14 to improve the uniformity of the broad area excitation and a long-pass filter 16 in front of the camera to block stray excitation light. The system may also include one or more filters 18 to select the wavelength range of the photo-excitation. With relatively thin samples and prior to metallisation of the rear surface of a PV cell, it is also possible to have the light source 6 and camera 10 on opposite sides of the sample 4 as shown in FIG. 2, in which case the sample itself can serve as a long-pass filter. However a long-pass filter 16 may still be required if a significant amount of stray excitation light, reflected for example off other components, is reaching the camera. Either way, one or more PL images can be acquired from a sample and analysed with a computer 20 using techniques disclosed for example in published PCT patent application Nos WO 2008/014537 A1, WO 2009/026661 A1 and WO 2009/121133 A1 to obtain information on average or spatially resolved values of a number of sample properties including minority carrier diffusion length, minority carrier lifetime, dislocation defects, impurities and shunts, amongst others, or on the incidence or growth of cracks. Importantly for fragile samples such as silicon wafers, the PL imaging technique is non-contact.
Early commercial systems were designed for laboratory use where total measurement times of order 10s are acceptable, while more recent innovations have led to line-scanning systems where wafers can be illuminated and imaged without interrupting their motion along a production line. As illustrated schematically in side view in FIG. 3, a line-scanning system typically includes beam shaping optics 22 to direct the excitation light 8 onto a sample of a semiconductor material 4, and collection optics 12 to image the emitted photoluminescence 2 onto an image capture device 24 such as a line camera, as well as various other components (homogenisation optics 14, long pass filter 16, excitation filter 18 and computer 20) as required, similar to the FIG. 1 system. The sample is moved through the measurement zone on transport belts 26 or rollers or the like, from left to right in this case as indicated by the arrow 28, so that the illuminated portion 30 and imaged portion 32 are scanned across the sample, and the line camera interrogated with an interrogation module (which may be part of or under the control of the computer 20) to build up a PL image of a substantial area of the sample. The illumination and imaging subsystems may for example be configured such that the total scanned area (i.e. the ‘substantial area’) corresponds to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100% of one surface of the sample. Preferably the illumination and imaging subsystems are configured such that the illuminated and imaged portions 30, 32 span the sample 4 as shown in FIG. 3A, enabling inspection of the entire area of one surface of the sample. The illuminated and imaged portions 30, 32 are typically oriented to be more or less perpendicular to the direction of motion 28, as shown in FIG. 3A.
It is also known in the art to use a time delay integration (TDI) camera instead of a line camera as the image capture device for detecting photoluminescence in a line-scanning system. A TDI camera can be thought of as an integrated array of line cameras, e.g. 96 or 128 lines of 1024, 2048, 4096 or 8192 pixels on a single chip, typically using the same silicon CCD technology as in conventional line or area cameras. TDI cameras are well suited for acquiring images of a moving sample, with the direction of movement perpendicular to the pixel lines; as the sample is moved the charge from the detected signal is transferred to the next pixel line and accumulated, with the charge transport and sample motion synchronised. Consequently, a TDI camera with N pixel lines measures the signal from a given portion of a sample N times, improving the signal-to-noise ratio by a factor of √N to N, depending on the dominant noise source, compared to a line camera for the same total measurement time. Similar to the configuration shown in FIG. 2, line-scanning systems can be designed with the light source and image capture device on opposite sides of the sample.
Localised regions of high series resistance (i.e. series resistance problems) are a common mode of PV cell failure or undesirably low efficiency, typically caused by defects that impede the transport of charge carriers. Such defects may for example include breaks in the metal contact structure, high contact resistance between the metal fingers or the rear contact and the respective silicon surface, and cracks in the silicon. Several luminescence-based techniques have been proposed, for example in published US patent application Nos 2009/0206287 A1, 2011/0012636 A1 and 2012/0113415 A1, for acquiring so-called series resistance images of PV cells or cell precursors, where local regions of excessive series resistance are identified via areas of higher or lower luminescence intensity. However these techniques require the acquisition of two or more luminescence images, or require making electrical contact to the cell, or both, and are not ideally suited to the rapid inspection of PV cells exiting a production line that currently may operate at up to 1800 or even 3600 wafers per hour.